US 7295582 B2
Thermo-optic tuning apparatuses and techniques that may be used in external cavity lasers are provided. A wavelength selection device may be placed in the external cavity and may include a thermally responsive substrate and tunable optical elements extending therefrom. Heating and heat monitoring actuators may be formed in the substrate itself, with the tunable optical elements being mechanically positioned in the recesses of the substrate for thermo-optic tuning. In some examples, a separate optical path length adjustment element is also placed in the thermally responsive substrate, or elsewhere, for separate frequency tuning. This adjustment element may be part of a control that is detangled from the normal servo controls used in laser tuning.
1. A laser apparatus comprising:
a gain medium coupled to the platform, the gain medium having a first output facet and a second output facet to emit a light beam from in response to an electrical input;
a reflector positioned to reflect the light beam and coupled to the platform, the reflector and the first output facet defining a laser cavity having an optical path length; and
a thermally responsive wavelength selection device positioned within the laser cavity and having a thermally responsive substrate mounted to the platform and a tunable optical element extending from the thermally responsive substrate into the laser cavity to adjust the optical path length in response to a thermal change in the thermally responsive substrate.
2. The laser apparatus of
3. The laser apparatus of
4. The laser apparatus of
5. The laser apparatus of
6. The laser apparatus of
7. The laser apparatus of
8. The laser apparatus of
9. The laser apparatus of
10. The laser apparatus of
11. The laser apparatus of
12. The laser apparatus of
13. A laser apparatus comprising:
a platform responsive to thermal changes induced in the platform to tune a frequency of an output beam of the laser apparatus;
a gain medium coupled to the platform, the gain medium having a first output facet and a second output facet to emit a light beam from in response to an electrical input;
a reflector positioned to reflect the light beam and coupled to the platform, the reflector and the first output facet defining a laser cavity having an optical path length;
a wavelength selection device positioned within the laser cavity and mounted to the platform for tuning the frequency of the output beam; and
an active optical path length adjustment device positioned within the laser cavity and coupled to a controller for tuning the frequency of the output beam in response to thermal changes induced in the optical path length adjustment device independent of the thermal changes induced in the platform to tune the frequency of the output beam.
14. The laser apparatus of
15. The laser apparatus of
16. The laser apparatus of
17. A method of laser operation comprising:
providing an external cavity laser having a gain medium with a first facet and a second facet for emitting a light beam into a laser cavity having an optical path length; and
positioning a wavelength selection device within the laser cavity, the wavelength selection device having a thermally responsive substrate mounted and a tunable optical element extending from the thermally responsive substrate into the laser cavity to adjust the optical path length in response to a thermal change in the thermally responsive substrate.
18. The method of laser operation of
19. The method of laser operation of
mounting the first etalon in a first recess within the substrate; and
mounting the second etalon in a second recess within the substrate.
20. The method of laser operation of
21. The method of
disposing a heater element in the thermally responsive substrate; and
disposing a temperature monitor element in the thermally responsive substrate.
22. The method of laser operation of
positioning a heating plane of the thermally responsive substrate substantially parallel to an optical axis of the laser cavity; and
positioning an incidence plane of the tunable optical element substantially orthogonal to the heating plane.
23. The method of laser operation of
positioning an optical path length adjustment device in the laser cavity, wherein the optical path length adjustment device is an active, thermo-optically controllable device; and
thermally isolating the optical path length adjustment device from the primary thermal tuning device.
Embodiments of the present disclosure relate generally to optoelectronic assemblies and, more particularly, to methods and apparatuses for facilitating tuning in a laser apparatus.
Fiberoptic telecommunications are continually subject to demands for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM). With a DWDM system many different and separate data streams may concurrently exist in a single optical fiber. Each data stream represents a different channel within the optical fiber, where each channel exists at a different channel wavelength. The modulated output beam of a laser operating at the desired channel wavelength creates the data stream. Multiple lasers are used to create multiple data streams, and these data streams are combined onto a single fiber for transmission in their respective channels.
The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers.
With the requirement for multiple tightly spaced channels, stable control over both the laser source and output frequency is important to system effectiveness. The lasers used in DWDM systems typically have been based on distributed feedback (DFB) lasers operating with a tuning etalon that defines the ITU wavelength grid. Due to manufacturing as well as performance limitations, DFB lasers are used as single channel lasers, or as lasers limited to tuning among a small number of adjacent channels. As a result, DWDM applications would require multiple different DFB lasers each at a different channel wavelength.
Continuously tunable external cavity lasers have been developed to overcome the limitations of DFB lasers. These lasers have a laser source and end mirror that define an external cavity used for wavelength tuning. For example, a tuning element within the external cavity, such as, an etalon device, is mounted to a support that is fixed to a platform extending between the laser source and end mirror. Controlling the temperature of the platform tunes the laser by altering the optical path length of the external cavity. Separate tuning between grid wavelengths may also be achieved by separately tuning the tuning element.
While such lasers have been used with some success, continuously tunable telecommunication lasers do present some design considerations. These lasers can be somewhat expensive and time consuming to manufacture. In particular, the tuning assemblies used in many continuously tunable lasers result from a lengthy sequential build-up process. During this process, certain components can only be fabricated after other components have been completed. Furthermore, once fabricated, the tuning speed of continuously tunable lasers is limited by the type of tuning method used in the assembly. More specifically, many current tuning methods require adjustment of multiple interdependent systems, which adds to control complexity and prolonged tuning times.
Example embodiments provide laser apparatuses and methods that use thermo-optic adjustment of an external laser cavity to provide wavelength stability and wavelength, or frequency, tuning. The laser apparatuses may include a gain medium coupled to a platform, the gain medium having an emitting facet for emitting a first light beam toward the external laser cavity. The external laser cavity may have a wavelength selection device for tuning the output wavelength of the laser apparatus, for example, the channel wavelength in a DWDM system. The external laser cavity, for example, may be mounted on a thermally adjustable substrate, where the responsiveness of the substrate may be controlled by a thermal control element and where thermal adjustments to the external cavity may be used to tune the laser apparatus. Although described below in certain example implementations, the wavelength selection component may comprise one or more etalons, gratings, prisms, filters or like devices, or various combinations thereof and may be tunable mechanically, electrically, thermally, or by other mechanism to provide wavelength selective feedback to the gain medium.
The wavelength selection component may be achieved via a hybridized device that includes a thermally responsive substrate with integrally mounted wavelength selection elements. The substrate may comprise a thermally conductive material to provide good thermal transfer to, and rapid temperature change of, the wavelength selection elements and other components in the external cavity thermally coupled to the substrate. Both the thermally responsive substrate and the wavelength selection elements may be simultaneously fabricated using known semiconductor processing to reduce overall assembly times. Both the substrate and the wavelength selection elements may be batched fabricated on semiconductor wafers, for example.
Some further examples of the laser apparatus may comprise a separate external cavity optical path length control device positioned in the light beam between the gain medium and the end reflector. The path length control device may be an active optical element positioned to provide independent frequency tuning for the laser apparatus. For example, the optical path length control device may be made of a thermally responsive optical material that changes in the refractive index under temperature variations. Unlike conventional passive optical path length control which relies on the laser apparatus's thermal tuning system, the optical path length control device may be independently controllable, detuning this control from the bulkier and slower laser apparatus's thermal tuning system that tunes an entire laser platform. The external cavity optical path length control, therefore, may be part of a faster, independent servo control for frequency tuning.
Although, various techniques are described in reference to particular illustrated examples, the descriptions are not limited to these examples. Further, while drawings are provided for illustration purposes, it is noted that the apparatuses may vary as to configuration and as to details of the parts, and that the methods may vary as to details and the order of events, without departing from the basic concepts as disclosed herein. Furthermore, although examples are disclosed primarily in an external cavity laser, the examples are not limited thereto. The relative sizes of components and distances therebetween as shown in the drawings may be exaggerated in some instances for reason of clarity, and should not be considered limiting. Further, any definitions herein are provided for reason of clarity, and should not be considered as limiting, and any technical and scientific terms used herein are intended to have the same meaning as commonly understood by those skilled in the art. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
The wavelength selection device 125 is mounted to the platform 116, which is coupled to a thermal control device such as a thermoelectric controller (TEC) 126, which controls the temperature of the entire platform 116 via thermal conduction. The TEC 126 therefore may be used to control the temperature of the platform 116 for temperature tuning of the components of the apparatus 110, where such temperature tuning may be used to adjust the optical path length of the laser apparatus 110.
The thermally conductive platform 116 may be made from any thermally conductive material. Various metals, metal nitrides, carbides and oxides, or alloys, blends, mixtures or composites thereof, can provide materials with good thermal conductivity and relatively low coefficient of thermal expansion (CTE). Aluminum nitride (AIN) may be used as the platform material in certain embodiments. The material of platform 116 may be selected to have a particular CTE that allows CTE matching to components mounted on platform 116. In some embodiments, platform 116 may comprise a high CTE material, such as aluminum, copper, zinc or other metal or metal alloy, so that a desired amount of physical expansion and contraction according to temperature control of the TEC 126 may occur. A controller 127 may control operation of the TEC 126. The platform 116 may in certain embodiments be made of silicon, allowing CTE matching to silicon components such as thermally tuned etalons. The use of silicon for platform 116 also allows for the integration of conductor paths directly onto the platform 116, and formation of specific geometries on platform 116 using conventional silicon machining and fabrication techniques.
The gain medium 112 is thermally coupled to platform 116 such that thermoelectric controller 126 can control the temperature of gain medium 112 by thermal conduction through platform 116. “Thermally coupling” as used herein means any mounting or coupling arrangement or configuration that provides effective heat flow for thermal control of thermally coupled components. Gain medium 112 is mounted on a thermally conductive carrier 128, which in turn is coupled to platform 116. Gain medium 112 thus is thermally coupled to platform 116 through carrier 128. Carrier 128, like platform 116, may comprise a thermally conductive material such as a metal, metal oxide, metal nitride, metal carbide, or alloys, blends, mixtures or composites thereof. In other examples, and by way of example not limitation, the carrier 128 may comprises aluminum nitride, silicon carbide, or a silicon carbide blend (alloy). In yet other examples, carrier 128 may comprise a copper tungsten (CuW) alloy. Further still, the substrate 116 and carrier 128 may be matched in CTE to each other and to gain medium 112. Thermally conductive adhesives or solders may be used to mount gain medium 112 onto carrier 128, and to mount carrier 128 and/or other various structures onto the platform 116.
The reflective element 114 may also be mounted on platform 116 as noted above and may, in certain embodiments, be thermally coupled to platform 116 using thermally conductive adhesives, solders and/or carriers or supports. The reflector 114 may be subject to temperature control during laser operation, but this is not required. A reflector may be mounted on platform 116 without thermal coupling thereto.
In addition to the overall frequency tuning via TEC 126, in the illustrated example, the wavelength selection device 125 is separately tunable. The wavelength selection device 125, for example, may include two optical elements in the form of etalons 130 and 132 extending from a thermally responsive substrate 134 mounted to the platform 116. This configuration is illustrated by way of example. The wavelength selection device 125 may comprise one or more optical elements, including for example etalons, gratings, prisms or other elements capable of providing feedback to gain medium 112 or otherwise useful in tuning an external laser cavity. The wavelength selection device 125 is shown positioned in optical path 119 between gain medium 112 and end reflector 114. The wavelength selection device 125 may be tunable such that the apparatus 110 has a single transmission peak within a wavelength range of interest such as, for example, the gain bandwidth of gain medium 124, the wavelength range of the ITU (International Telecommunications Union) “C” band (approximately 192.1 THz to approximately 196.1 THz or approximately 1525 to approximately 1565 nanometers), or other wavelength range.
In addition to controlling the gain medium 112, the controller 127, or other control element, may control the wavelength selection device 125, e.g., by operative coupling to wavelength selection device 125 to provide for adjustment or selection of the wavelength of the transmission peak defined by wavelength selection device 125, and, hence the wavelength of light that is fed back to gain medium 112. Example control elements, and other examples of some of the illustrated features are also described in U.S. Pat. No. 6,763,047, which is incorporated herein by reference.
The first etalon 130 includes faces 200, 202, and has a first free spectral range (FSR) according to the spacing between faces 200, 202 and the refractive index. Second etalon 132 includes faces 204, 206, and has a second FSR according to the spacing between faces 204, 206 and the refractive index of the material of etalon 132. The etalons 130, 132 may comprise the same material or different materials with different refractive indices. Further, the etalons 130, 132 may be tunable by adjustment of their optical thickness, to provide for adjustment or tuning of the FSR of each, which in turn provides selective wavelength tuning for the laser apparatus 110 as described further below. Tuning of etalons 130, 132 can involve adjustment of the distance between faces 200, 202 and 204, 206 of etalons 130, 132 respectively and/or adjustment of the refractive index of the etalon material. Various tuning mechanisms involving various techniques may be used, including thermo-optic, electro-optic, acousto-optic, piezo-optic, mechanical, or other tuning to vary refractive index of etalon material and/or vary the spacing of etalon faces. And more than one such tuning effect may be applied simultaneously to one or both etalons 130, 132, depending upon the particular embodiment of the invention.
In the illustrated example, the etalons 130, 132 each are thermo-optically tunable, where the term “thermo-optic” tuning means tuning by temperature-induced change in etalon material refractive index, temperature induced change in the physical thickness of an etalon, or both.
To thermo-optically tune the etalons 130, 132 separately from TEC tuning, each are disposed in or on the thermally responsive substrate 134. In the illustrated example, etalon 130 is mounted in a first recess 210 in the substrate 134, while the etalon 132 is mounted in a second recess 212. In the illustrated example, the first recess 210 is positioned within a first heating region 214 and the second recess is positioned in a separate heating region 216. In this example configuration, each of the etalons 130, 132 may be separately thermo-optically tuned by separately controlling the heating within the regions 214 and 216. To facilitate thermal isolation between the two regions 214, 216, a thermal barrier 218 separates the regions, thereby acting as a thermal ground to a heat sink, platform 116, or other mounting substrate upon which the device 125 may be engaged during operation. The thickness and composition of the barrier may be adjusted to affect the desired isolation to thereby provide sufficient thermal cross-talk isolation. In the illustrated example, each recess the heating region 214, 216 is formed within the substrate 134 such that the entire substrate 134 surrounds these regions with a thermal ground boundary 220 of which the barrier 218 is grounded thereto.
The illustrated configuration shows a hybrid wavelength selection component assembly, in which the active tuning structure, the substrate 134, is separated from tuned elements, the etalons 130, 132. This may have the advantage over conventional techniques of allowing simultaneous processing of the two structures, thereby reducing fabrication and assembly times over monolithic build-up assemblies. The etalons 130, 132 may be batch fabricated on one wafer and the substrates 134 may be batch fabricated on a different wafer, although alternatively sets of each may be fabricated together on a single wafer. The substrate 134 may be formed from known techniques, including silicon etching and growth processing techniques like hydrofluoric acid (HFC) fabrication, where a glass substrate is direct written with a laser scan across the substrate 134, exposed to ultraviolet light, and then exposed to an acid wash to remove the non-patterned areas and leave the recesses 210, 212. In some examples, the substrate 134 is formed of a material having sufficient thermal properties to allow for uniform and time responsive heating, while being mechanically strong enough to withstand an assembly process that mounts the etalons 130, 132 to the substrate 134.
The etalon materials used in certain embodiments have temperature dependent refractive indices as well as coefficients of thermal expansion such that thermo-optic tuning involves simultaneous thermal control of etalon material refractive index as well as thermal control of etalon physical thickness by selective heating or cooling. Semiconductor materials such as silicon and gallium arsenide provide a large change in refractive index with temperature and may be used for the material etalons for effective thermo-optic tuning. In some examples, the etalons 130, 132 may comprise silicon, which is transparent over the gain bandwidth or output wavelength range of InGaAs, InGaAsP and other gain media. Other materials include liquid crystal polymers (LCPs), or materials formed of inert, no-hydroscopic materials. The etalons 130, 132 may be formed of the same or different materials from that of the substrate 134. In many examples, the materials used in each would have similar or identical thermal expansion coefficients. With the substrate 134 and etalons 130, 132 formed, and by way of example, not limitation, the wavelength selection component 125 may be formed by using a pick-and-place assembly process that mounts the etalons 130, 132 into the recesses 210, 212, respectively. Once mounted, the etalons 130, 132 may be affixed in place by any epoxy, solder, or other adhesive.
The substrate 134 may be controlled by a controller such as the controller 127, e.g., a conventional data processor, and provides tuning signals to device 125 for thermal adjustment or tuning of etalons 130, 132 according to selectable wavelength information stored in a look-up table or other wavelength selection criteria. The substrate 134 may include both heating and thermal monitoring elements, for example, in the form of a heater and a resistive thermal device (RTD) that is used to monitor temperature conditions on or within the substrate 134, examples of which are shown and discussed in reference to
For the illustrated example, four wire bond pads 300-306 are used to monitor heat in region 214, with wire bond pads 300 and 302 coupled to monitoring leads 308 and 310, respectively, and wire bond pads 304 and 306 coupled to monitoring element leads 312 and 314, respectively. The element leads 308-314 are shown by way of example. One pair (e.g., input/output pads 300 and 306) may be used to carry constant current, while the other input/output pair (e.g., pads 302 and 304) may be used to probe a voltage drop or change. The monitored data may be provided to the controller to calibrate the tuning operation of the laser or as part of the servo control of the tuning operation.
While the wire bond pads 300-306 are used in monitoring, wire bond pads 320 and 322 are used to form a heater for thermo-optic tuning. The bond pads 320 and 322 are coupled to heating elements 324 and 326, respectively, for uniform heating. The heating elements 324 and 326, for example, may be patterned to affect uniform temperature changes for the etalon 130. Though not discussed in detail, the etalon 132 may have similar monitoring and heating wire bond regions 328 and 330, with accompany elements. To save a bond pad, or pin, and reduce electric circuit complexity, the constant current output pad (e.g., pad 306) may be shortened to the input for the constant current pad for the second temperature controlled region, i.e., for etalon 132 in
The configurations in
The substrate 134 may be adjusted to form an operable thermal responsiveness that evenly heats the etalons, or other tunable elements, and that has sufficient thermal responsiveness to perform such heating at relatively low power requirements. The materials and dimensions of the substrates 134, for example, may be formed of materials having relatively small thermal expansion coefficients to allow for more uniform heating. The use of materials with low CTEs may avoid strain in the joint between substrate and etalon. Otherwise, strain can manifest as a non-uniform optical path length across the aperture of the etalon, and, if sufficiently high, can lead to unreliability of the joint over the lift of the product. Additionally, the material may be one that has a good thermal response, by way of example not limitation, a response of greater than 1200 K per input Watt of electrical power to the heating element leads. A 25 μm thick substrate, along the Z-axis, for example, may be heated to 70° C. using only 50 mW of input power, thereby resulting in a thermal responsiveness of 3400 K/Watts. Similar conditions for a 100 μm thick substrate could result in a 2650 K/Watt thermal responsiveness, again by way of example. Thus, the substrates 314 and 500 may be formed to operate within the 1 Watt box power usage requirement for some tunable laser devices.
The illustrated data is provided by way of example and shows suitably fast response times for the configuration wavelength selection device. Thermal time constants below 100 ms are achievable, which is sufficient to allow for fast multi-channel tuning and testing of the laser apparatus, as may be useful in tunable transponder DWDM applications.
The wavelength selection device may be a hybrid assembly, formed of mounting tunable elements into a micro-hotplate substrate. During mounting of etalons, for example, into a substrate, substantial forces may be applied to the substrate during a pick-and-place assembly. These forces although primarily along the Z-axis and into the substrate may also result in lateral forces along the XY-plane of the substrate. To provide additional support during assembly, a substrate may be supported by a localized mounting structure that provides an equal and opposite force to that applied to the substrate during assembly and that prevents substantial amounts of lateral force transfers from the Z-axis insertion force. For example,
The mounting structures 604 and 606 are cylindrical in the illustrated example, and are able to counter the load compression load without transferring detrimental lateral forces, which can more easily harm the substrate, due to its lattice orientation and differential between compression and later mechanical strength. Although shown as cylindrical in shape, the shape of the mounting structures are not limited thereto.
During tuning of existing laser apparatus, a TEC could be used for thermal frequency tuning, but that tuning would affect all components on the TEC, including the laser source, end mirror, and the wavelength selection device. The laser apparatus 110 provides an example where etalon tuning may be isolated from such TEC tuning, for example, by forming the thermally responsive substrate to be thick enough to isolate a heating plane of the substrate from the undersurface TEC. Additional isolation of laser apparatus components to allow for detangle tuning and operation are also contemplated.
A few example formulations will now be discussed. The tuning rate for silicon, in the telecom C-band, is about ˜10 GHz/K. This rate of change is driven by the thermo-optic coefficient (dn/dT) primarily, with a small contribution (for silicon) from the CTE. The fractional change of the optical path length may be expressed by d(OPL)/OPL equaling (1/n)dn/dT+CTE. The temperature change necessary to hop between Vernier channels also depends on Joint Free Spectral Range (JFSR) and average free spectral range chosen when specifying the pair of etalons. For an example, a JFSR of 8.25 THz and an average free spectral range of 275 GHz, a temperature rise of 275 GHz/(10 GHz/K)=27.5 K could be used to address any channel within the JFSR (design tuning range), and a change in the temperature difference between the two etalons of ˜27.5 K/8.3 THz/275 GHz)=0.917 K.
Focusing on the optical path length adjust, without temperature influence, the following formula may be used to lock a laser based on changes to the optical path length:
In the illustrated example, the element 712 is an optical spacer having a first optical surface 714 and a second optical surface 716, which each may be coated with an AR coating to prevent internal reflections within the element 712. Additionally or separately, the element 712 may be tilted about the Z-axis and/or rotated in the XY-plane approximately 1° to 2° to reduce the spatial overlap of the reflection and hence the consequential spectral fringes measured in a single (spatial) mode waveguide. Further, although the element 712 is shown with parallel surfaces, alternatively it may have a wedge shape to reduce angular overlap of the fringes and likewise reduce spectral fringes. The element 712 may be formed of the same materials as used to form etalons forming the wavelength selection device 704, when etalons are used. For example, as an optical spacer, the element 712 may be formed of silicon, which is transparent over the gain bandwidth or output wavelength range of InGaAs, InGaAsP and other gain media. Other materials include liquid crystal polymers (LCPs), or materials formed of inert, no-hydroscopic materials.
The element 712 may be positioned directly on the platform 708 or mounted via a mount 718. The mount 718 may be formed of a material for thermal isolation or another material for thermal isolation. In some configurations, the element 712 may be an optical spacer with monitoring (or RTD) electrodes 720 a, 720 b, 722 a, 722 b and heating electrodes 724 a, 724 b, 726 a, and 726 b, as shown in
Alternatively, the element 712 may be a thermally conducting optical element disposed within a thermally responsive substrate, similar to the substrate 134.
The configuration of
Each of these controls 910-914 are provided by way of example. It will be understood by persons skilled in the art that the controls can have different configurations, including additional, fewer or other elements. In the illustrated example, the controls 910-914 may provide a feedback to the controller 904, thus representing the interdependence of the TEC control on such values; as the TEC is changed, the static conditions measured by the controls 910-914 may change. The use of a separate active optical path length adjustment component in control 910, however, detangles at least one of the optical path length adjustment control mechanisms (e.g., that of an optical spacer) from TEC tuning. That is, although a TEC may still be adjusted to control for optical path length, with an independently controllable optical path length adjustment device that can achieve frequency tuning, the TEC need not be adjusted during a frequency tuning. Instead, the TEC may be held a constant or relatively constant temperature and the laser apparatus may still achieve frequency tuning via the active frequency tuning optical path length adjustment. With detangling that adjustment, which before had only been used for dithering control and could not perform frequency tuning, a laser apparatus may be capable of frequency tuning at a much faster rate due to the lower thermal mass of the adjusting element. In some examples, the improvements in frequency tuning times may an order of magnitude or more.
Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalence.